This invention relates to an oscillator circuit. More particularly, the present invention relates to an oscillator circuit which generates an oscillation output signal having a frequency of n times (n being two or more integer) an input signal frequency.
A frequency-multiplication circuit using a tuning circuit and an oscillator circuit utilizing the phase lock loop (hereinafter referred to as "PLL") technique have been known as a circuit for obtaining a signal having a frequency which is an integer times as high as the frequency of an input signal.
The multiplication circuit using a tuning circuit is such that an input signal waveform is distorted to generate secondary, tertiary and higher harmonic signal components. One of the components is picked up by the tuning circuit. The multiplication circuit is advantageous in that the circuit arrangement is relatively simple. However, the multiplication circuit is not free from the problems that the voltage level of an output signal is changed and the phase relationship between the input and output signals can not be designed accurately. There is another disadvantage when the input signal is interrupted, because the output signal is no longer generated.
The oscillator circuit utilizing the PLL technique comprises a free-running oscillator of a voltage control type, a frequency divider, a phase comparator and a low pass filter. The frequency divider frequency-divides the oscillation signal of the free-run oscillator. The phase of a signal from the divider is compared with the phase of an input signal by the phase comparator, to generate a signal corresponding to the phase error. This signal is applied to the free-running oscillator through the low pass filter. Responsive thereto, the oscillation frequency of the free-running oscillator is controlled so that the phase error becomes zero.
Hence, the oscillation frequency of the free-running oscillator can be set to any desired frequency which is an integer times as high as the input signal frequency. The frequency set is made by selecting an appropriate division ratio of the frequency divider. This oscillator circuit is advantageous in that the output level and the phase relationship between the input and output signals can be set with an extremely high level of accuracy. The free-running oscillation output can be obtained from the free-running oscillator even when the input signal is interrupted.
However, the circuit construction is very complicated, as described above, resulting in a high production cost. In addition, the low pass filter requires a large time constant. Therefore, it cannot be fabricated in a semiconductor monolithic integrated circuit device.